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Acoustic Power Transfer (APT) is a wireless power transmission technology that uses sound waves, typically in the ultrasonic frequency range (above 20 kHz), to deliver energy over a physical medium such as air, water or solids. Unlike electromagnetic wireless power systems, APT is not affected by EMI  and is especially useful for applications in biomedical implants, underwater systems, and metal-enclosed environments. Not only can APT be used to power systems, it also can transfer data at the same time. 
Acoustic waves are used for transmitting energy. Figure 3 shows a typical APT system consisting of a pair of acoustic transducers separated by a medium. The power transfer generally consists of three stages:

1.	Electrical to acoustic conversion: a piezoelectric transducer converts the electrical power into mechanical vibrations using the piezoelectric effect. The alternating high frequency voltage, provided by the power converter, is applied across a piezoelectric element causes it to oscillate. This generates ultrasound waves at the same frequency.

2.	Acoustic wave propagation: the mechanical vibration moves through a medium as longitudinal  acoustic wave. These waves carry energy and can be directed to maximize transmission efficiency.

3.	Acoustic to electrical conversion: a second piezoelectric transducer picks up the acoustic waves and converts them back into electrical energy, which can be rectified and regulated for powering a device or charging a storage unit.

A main advantage of APT over electromagnetic based WPT like inductive power transfer, is the lower wave speed of acoustic waves compared to electromagnetic waves. It is typically five to six times slower than the speed of light. This allows the operating frequency of an APT system to be reduced by the same factor for a given wavelength, while maintaining similar directionality. This makes the system becomes simpler and energy losses are reduced. Additionally, APT is suitable for applications where EM-based WPT cannot be used, such as environments with metal shielding. 
The acoustic impedance of the medium is a very important value that needs to be checked. Acoustic impedance Z is defined as the ratio between the acoustic pressure p and the particle velocity v.

The efficiency of APT is highest when the acoustic impedance of transducers matches that of the medium. It minimizes reflection and allows the maximum amount of acoustic energy to be transferred. When there is a mismatch, a significant portion of the wave is reflected. This will reduce the overall efficiency. There are several methods to match mismatched impedances:
1.	Using a matching layer: the use of a matching layer reduces the overall reflection of the waves. The acoustic impedance of that layer is calculated by taking the mean between the transducer and the medium impedance. The thickness of that layer should be a quarter of the wavelength of the material used in the matching layer.

2.	Change the transducer: select piezo materials with an acoustic impedance closer to the medium.

3.	Tune operating frequency: matching of the acoustic impedances is easier when the APT is operating at resonance frequency.

4.	Use coupling materials: applying gels, water… between the transducer and the medium helps reducing the reflection. This is for instance used in an echography.

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